Opinion: Electric WH w/ Heat Exchanger

Hi, Just looking for opinions here. What would the potential problems and / or downsides be to installing a closed loop heat exchange system in my existing ductwork and using an 80 gallon electric water heater to heat it? This would not replace my heat pumps obviously, it would just be a supplemental heat. I would obviously need a circ pump and expansion tank and I am guessing I would have to bleed air out of tank and add water due to evaporation ever so often? My thinking is; the air from the vents would be warmer than heat pump air, it would heat the house faster, and therefore the HVAC would not run as much, therefore saving some electricity. Any thoughts? Thanks

Advertisement

COP, co-efficient of performance. Heat pump will give you more heat/dollar of electricity than straight electric heat which is what you get from a water heater. Google air to air heat pump and readup about it.

Find out the wattage of the hw tank elements. Probably around 5000 watts which is rather small. Probably have 10-15 kw strip heaters for aux heat with the heat pump. Doubt that 5000 watts would do much good or increase the temp very high. Then there is a lot of loss in the pipes due to convection/radiation and the cost to run a circ pump. May warm the air a bit but at a large cost IMO.

It will also erode the tank and cause it to leak soon, those tanks are not very thick and not designed for continuos flow/circulation.

It will however though, use more electric to heat your house. Then just heating with the heat pump.

EG: A 13 SEER 3ton heat pump operating at an outdoor temp of 30 with a COP of 2.8, and an 80 gallon electric water with coil piped into the supply. With the house requiring 30,252 BTUs an hour.

With the heat pump and water both providing heat at the same time. It would take 41 minutes to heat the house. With the water hater using 3.12KWs, and the heat pump using 1.95KWs, for a total of 5.07 KWs consumed.

Using only the heat pump. it would take the heat pump 60 minutes. And the heat pump would only consume 2.8 KWs total.

So using the water heater would increase your heating bill by 45% over using the heat pump alone. When the heat pump can maintain the house temp.

It seems that if I have 80 gallons of 150 degree water, it would heat the air better than what the heat pump does when the outside temp is below freezing. Especially since the air would be preheated by the heat pump before it goes through the water to air exchange. The tank erosion thing is the type of thing I am looking for though. Why would it erode and can it be prevented? I wasn't planning on going into the backstory, but here it is anyway. It is a 4200sf house with two electric heat pump systems. 9 months out of the year, all is good. But from Dec to Feb, my systems pretty much run 24 hours a day. With all the outdoor unit defrost cycles and the aux heat coming on, my bills get to over $500 a month for those three months. I have real time amp meters on my panels and I can see what my amp draw is per panel and per phase at any time. The water heater pulls around 18 amps. When the heat pump is running normally, it is pulling around 20 amps. When the defrost cycle and aux heat are running, it is running at about 80 amps. That is just for the downstairs unit. If I could reduce the amount of defrost cycles and get the unit to actually shut off from time to time, I am pretty sure I could reduce my kw usage. Last Jan. we used over 6000 kw hours. We have spent around $9k on our heat systems in the last 2 years and they are only 6 years old. I am guessing that it is due to the continuous running in the cold months. I would put in an outdoor wood furnace, but we are in a subdivision and I don't think the neighbors would approve of the smoke. Thanks for the replies, Understand I am just brainstorming here.

I'm going to take a wold guess that you have 2-4 ton heat pumps. or a combination of sizes that adds up to atleast 8 tons. And they are attached to duct work that is sized for the next tonnage smaller.

if the house isn't getting warm enough. then there is probably a problem with your strip heaters.

Actually. Many thermostats are set up to just cycle the aux heat on and off, but keep the heat pump running 24/7 after it gets so cold outside.

Didn't know that. I guess that could be it. We just replaced one entire system with a 15 SEER unit. I don't know if it will be any different this winter, but I guess we will find out. I just like to explore all my options.

HW tanks have 6-8 yr gurantees. After producing billions of them the manufacturers know just the right thickness of metal to make them last that long. Depending on your water quality etc there is always some erosion when water circulates. Commercial tanks are thicker but a lot more expensive. If you add a water coil inside your ductwork you will add resistance to the air flow/increase your static pressure and create another problem.